fix a bug on calling the dontcare value in dataset

[CharlesAuthier]

Fixing a bug about the dontcare value from the segmentation dataset, this value is call in train_segmentation line 411 and 416. The value was not set in the dataset I presume it was remove in an other PR and the code didn't follow, but this value is needed if we want to have the iou and the report_classification save in mlflow or other trackers.